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Evaluation of Numerical Flow and Dispersion Simulations for Evaluation of Numerical Flow and Dispersion Simulations for Street Canyons with avenue-like Tree Planting by Comparison Street Canyons with avenue-like Tree Planting by Comparison
with Wind Tunnel Datawith Wind Tunnel Data
CHRISTOF GROMKE Institute for Hydromechanics - University of Karlsruhe (GERMANY) gromke@slf.ch
RICCARDO BUCCOLIERI Dipartimento di Scienza dei Materiali - University of Lecce (ITALY) riccardo.buccolieri@unile.it
SILVANA DI SABATINO Dipartimento di Scienza dei Materiali - University of Lecce (ITALY) silvana.disabatino@unile.it
BODO RUCK Institute for Hydromechanics - University of Karlsruhe (GERMANY) ruck@uka.de
Cavtat (Croatia) - October 6th-9th, 2008
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for
Regulatory Purposes
UNIVERSITY OF SALENTO (ITALY)
UNIVERSITY OF KARLSRUHE (GERMANY)
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
• Introduction - flow and dispersion in street canyon - influence of trees: wind tunnel and numerical works in literature
• Approach - combination of wind tunnel and CFD modeling
• Results - experimental and numerical results/comparison - discussion and comparison with previous works
• Summary and Conclusions
Outline
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
IntroductionDispersion in Urban Street Canyons
basic unit forming a citywhere people and traffic are geometries which affect flow, turbulence fields and dispersionwhere trees can be planted
Street canyon
3-D flow field inside a street canyon with aspect ratio H/W ≈ 0.5
• approaching flow perpendicular to street axis
• long street canyons (L/H > 7)
• two dominating vortex structures: canyon vortex, corner vortex
• superposition of vortex structures
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
idealization of street canyon geometry
essential geometrical structures
isolated urban street canyon
urban street canyon
Idealization
Modeling street canyon model – wind tunnel
IntroductionUrban Street Canyon and Modeling
street canyon model– CFD
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
What is influence of What is influence of treestrees on on
the flow field, natural ventilation and therefore local concentration?the flow field, natural ventilation and therefore local concentration?
IntroductionStreet Canyons with Tree Planting
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
NUMERICAL INVESTIGATIONS•Gross G. (1987):
•influence tree planting (two rows arranged sidewise of the street next to the building walls) •k-ε turbulence closure scheme and modelling of the tree crowns by porous bodies•decelerated flow velocities near the building walls and increased pollutant concentrations inside the street canyon
•In a similar arrangement, Ries, K. and J. Eichhorn (2001) found local increases of the pollution concentration at the leeward wall accompanied by reduced flow velocities due to trees.
IntroductionStreet Canyons with Tree Planting - literature
2D models applied (do not account for the highly 3D flow fields present in real urban street canyons of finite length)
Gross, G., 1987: A numerical study of the air flow within and around a single tree. Boundary Layer Meteorology, 40, 311-327.Ries, K. and J. Eichhorn, 2001: Simulation of effects of vegetation on the dispersion of pollutants in street canyons. Meteorologische Zeitschrift, 10, 229-233.
Impact of trees in street canyons on pollutant dispersion not widely considered
Both experimental and numerical investigations are present in literature
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
IntroductionStreet Canyons with Tree Planting - literature
WIND TUNNEL INVESTIGATIONSGromke, C. and B. Ruck, 2007: Influence of trees on the dispersion of pollutants in an urban street canyon - Experimental investigation of the flow and concentration field. Atmospheric Environment, 41, 3287-3302Gromke, C. and B. Ruck, in press: On the impact of trees on dispersion processes of traffic emissions in street canyons. UAQ2007 Special Issue in Boundary Layer Meteorology, accepted for publicationGromke, C. and B. Ruck, 2008: Aerodynamic modeling of trees for small scale wind tunnel studies. Special Issue on Wind and Trees in Forestry, 81, 243 – 258
Institute for Hydromechanics - University of Karlsruhe (GERMANY)
Flow and concentration fields in urban street canyons of different aspect ratios with various avenue-like tree planting configurations
Tree planting characteristics: influence of crown shape, diameter, height, porosity and planting density
FLOW: air exchange and entrainment conditions considerably modified, resulting in lower flow velocities and in overall larger pollutant charges inside the canyon.
DISPERSION: increases in pollutant concentrations at the leeward and decreases at the windward
street canyon with miscellaneous tree arrangements
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
COMPARISON WT – CFD INVESTIGATIONS (H/W=1)
IntroductionStreet Canyons with Tree Planting – our previous work
single block: impermeable
single block: permeable
Empty street canyon
Street canyon with tree planting along the center axis
Gromke, C., R. Buccolieri, S. Di Sabatino and B. Ruck, 2008: Dispersion study in a street canyon with tree planting by means of wind tunnel and numerical investigations - Evaluation of CFD data with experimental data. Atmospheric Environment. DOI:10.1016/j.atmosenv.2008.08.019
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
IntroductionStreet Canyons with Tree Planting – our previous work
- 0 . 4 - 0 . 2 0 0 . 2 0 . 4
x / H
L D V M e a s u r e m e n t
0
0 . 2
0 . 4
0 . 6
0 . 8
1
1 . 2
z/H
- 0 . 4 - 0 . 2 0 0 . 2 0 . 4
x / H
k - e p s i l o n T M
0
0 . 2
0 . 4
0 . 6
0 . 8
1
1 . 2
z/H
- 0 . 4 - 0 . 2 0 0 . 2 0 . 4
x / H
R S M T M
0
0 . 2
0 . 4
0 . 6
0 . 8
1
1 . 2
z/H
w velocities – middle of the canyon
- 0 . 4 - 0 . 2 0 0 . 2 0 . 4
x / H
k - e p s i l o n T M
0
0 . 2
0 . 4
0 . 6
0 . 8
1
1 . 2
z/H
- 0 . 4 - 0 . 2 0 0 . 2 0 . 4
x / H
R S M T M
0
0 . 2
0 . 4
0 . 6
0 . 8
1
1 . 2
z/H
- 0 . 4 - 0 . 2 0 0 . 2 0 . 4
x / H
L D V M e a s u r e m e n t
0
0 . 2
0 . 4
0 . 6
0 . 8
1
1 . 2
z/H
empty street canyon permeable crown
•slightly smaller flow velocities in the upward •significantly lower velocities in the downward moving part•volume flow crossing the horizontal plane at z/H = 0.7 is reduced (-36 %)
canyon vortex
Air volume rotating in canyon vortex is reduced in the presence of tree plantings
CFD simulations with FLUENT result in higher pollutant concentrations and lower flow velocities inside the street canyon in comparison with experimental investigations. We need to diminish the Schmidt number
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
• Concentration and velocity profiles and contours - Comparison with wind tunnel data
• Approaching flow perpendicular to street axis
ApproachOutline of Investigated Tree Planting Configurations
Empty street canyon - H/W=0.5
Street canyon with tree planting
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
W = 36 m
A B
L = 180 m
H = 18 mu(z)
α = 0.30
traffic lane
model trees
concentrationmeasuringtaps
roughnesselements
line source
z
y
x
Boundary layer wind tunnel
• closed, circulating BLWT
• vortex generators and roughness elements
• power law profile exponent α = 0.30
• uδ = 7 m/s
• Reynolds-No. Re = 37.000
Urban street canyon model (scale 1:150)
• isolated long street canyon (H/W = 0.5, L/W = 10)
• line source at street level
• tracer gas (sulfur hexafluoride SF6)
• 126 measurement taps at canyon walls
ApproachExperimental Investigations in a Boundary Layer Wind Tunnel
(Full scale)
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
Concentration Measurements
• Electron Capture Detector (ECD) - model Meltron
LH 108
• measurement of mean tracer gas concentrations
(sulfur hexafluoride SF6)
• averaging time for one concentration measurement:
about 120 sec
• 14 taps measured simultaneously
• 3 to 4 hours for each configuration, including the gas
analysis
lQ
Hucc
T
refm=+
cm measured concentration
uref reference velocity
H building height
QT/l strength of line source
ApproachMeasurement Instrumentation
Dimensionless concentrations c+
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
pressure loss coefficient determined in forced convection conditions (to describe the aerodynamic characteristics)
two crown porosities (typical for crown porosities of deciduous trees):
Pore volume fractions of PVol = 97.5 % (λ = 80 m-1, LOOSELY FILLED) PVol = 96 % (λ = 200 m-1, DENSELY FILLED)
ApproachOutline of Investigated Tree Planting Configurations
WT modelling of porous tree crowns
custom-made lattice cages aligned symmetrically along the street axis
cages divided into 31 cells, filled with filament/fibre-like synthetic wadding material
duρ
pp
dp
pλ leewardwindward
dyn
stat2)21(
Δ −==
Δpstat: difference in static pressure windward and leeward of the porous obstaclepdyn: dynamic pressure, u: mean wind velocityd: porous obstacle thickness in streamwise length
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
Description of FLUENT simulation setup
• commercial CFD-Code
• RANS-Equations
• turbulence closure schemes
- RSM
• second order discretization schemes
• grid: hexahedral elements- ~ 400,000
- δx=0.05H, δy=0.25H, δz=0.05H
• turbulent Schmidt number Sct = 0.7
ν=ydiffusivitturbulent
itycosvisturbulent
DSc
t
tt
ApproachNumerical Investigations - CFD Simulations with FLUENT
uH=4.76 m/s: undisturbed wind speed at the building height H
α=0.30: power law exponent
u*=0.52 m/s: friction velocity
κ=0.40: von Kàrmàn constant
Cμ = 0.09
α
=
H
z
u
zu
H
)()
δ
z(
C
uk
μ
−= ∗ 12
)δ
z(
κz
uε −= ∗ 1
3
INLET
30H8H
8H
INFLOW
OU
TFL
OW
SYMMETRY
WIND
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
ApproachOutline of Investigated Tree Planting Configurations
CFD modelling of porous tree crownsA cell zone is defined in which the porous media model is applied and the pressure loss in the flow is determined
The porous media model adds a momentum sink in the governing momentum equations:
This momentum sink contributes to the pressure gradient in the porous cell, creating a pressure drop that is proportional to the fluid velocity (or velocity squared) in the cell.
The standard conservation equations for turbulence quantities is solved in the porous medium. Turbulence in the medium is treated as though the solid medium has no effect on the turbulence generation or dissipation rates.
viscous loss term + inertial loss term
Si: source term for the i-th (x, y, or z) momentum equation : magnitude of the velocity D and C: prescribed matricesv
permeable zone with the same loss coefficient λ as in wind tunnel experiments
LOOSELY FILLED: λ = 80 m-1
DENSELY FILLED: λ = 200 m-1
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
ResultsEmpty Street Canyon
Largest concentrations at the pedestrian level in proximity of wall A
Wall A shows larger concentrations than windward wall B (about 3 times larger)
Decreasing concentrations towards the street ends
Wall A Wall B
WT CONCENTRATIONSWT CONCENTRATIONS
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
WIND
ResultsEmpty Street Canyon
CFD FLOWCFD FLOW
Wall A shows larger concentrations than windward wall B: dominating vortex-like structure around the street canyon centre
Decreasing concentrations towards the street ends: enhanced natural ventilation at the street canyon ends, where air exchange is not only provided by vertical exchange with the above roof flow but also with entering flow laterally
x / H
z/H
- 1 - 0 . 6 - 0 . 2 0 . 2 0 . 6 10
0 . 2
0 . 4
0 . 6
0 . 8
1
1 . 2
0 . 1
0 . 2
0 . 3
0 . 4
-1 -0.5 0 0.5 1-6
-5
-4
-3
-2
-1
0
1
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0.1
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0.9
1
y /H
x/H
WIND
z=0.5Hy=1.25H
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
ResultsEmpty Street Canyon
CFD - WT CONCENTRATIONSCFD - WT CONCENTRATIONSWall A Wall B
CFD concentration pattern qualitatively similar to that obtained in the wind tunnel
Slight underestimation of the measured concentrations in proximity of wall A
calculated concentrations relative deviations [%] in respect of measurements
street canyon model – wind tunnel
street canyon model – CFD
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
Wall A Wall B
relative deviations [%] in respect of tree-less street canyon
Increases in concentrations in proximity of wall A and decreases near wall B
Maximum concentrations at pedestrian level in proximity of wall A
Differently to the tree-free street canyon case, less direct transport of pollutants from wall A to wall B occurs
WT CONCENTRATIONSWT CONCENTRATIONS
ResultsStreet Canyon with Tree PlantingLoosely filled CrownLoosely filled Crown
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
Increases in concentrations in proximity of wall A and decreases near wall B
The pollutants are advected towards the leeward wall A, but, since the circulating fluid mass is reduced in the presence of tree planting, the concentration in the uprising part of the canyon vortex in front of wall A is larger
ResultsStreet Canyon with Tree Planting
CFD FLOWCFD FLOW
- 1 - 0 . 6 - 0 . 2 0 . 2 0 . 6 10
0 . 2
0 . 4
0 . 6
0 . 8
1
1 . 2
x / H
z/H
0 . 1
0 . 2
0 . 3
0 . 4
-1 -0.5 0 0.5 1-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
y/H
x/H
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
x / H
z/H
- 1 - 0 . 6 - 0 . 2 0 . 2 0 . 6 10
0 . 2
0 . 4
0 . 6
0 . 8
1
1 . 2
0 . 1
0 . 2
0 . 3
0 . 4
-1 -0.5 0 0.5 1-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
0.1
0.2
0.3
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0.5
0.6
0.7
0.8
0.9
1
y/H
x/H
Differently to the tree-free street canyon case, less direct transport of pollutants from wall A to wall B occurs
Most of the uprising canyon vortex is intruded into the flow above the roof level. Here, it is diluted before partially re-entrained into the canyon. As a consequence, lower traffic exhaust concentrations are present in proximity of wall B
WIND
WIND z=0.5Hy=1.25H
Loosely filled CrownLoosely filled Crown
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
CFD - WT CONCENTRATIONSCFD - WT CONCENTRATIONS
ResultsStreet Canyon with Tree Planting
calculated concentrations relative deviations [%] in respect of measurements
FLUENT is successful in predicting an increase in concentrations in proximity of wall A and a decrease near wall B and the relative deviations in respect of tree-less street canyon
As in the tree-free case, it slightly underestimated experimental data
Wall A Wall B
street canyon model – wind tunnel
street canyon model – CFD
Loosely filled CrownLoosely filled Crown
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
In comparison to the street canyon with the tree planting of loosely filled crown, no essential changes are found both in wind tunnel experiments and CFD simulations
The degree of crown porosity is of minor relevance for flow and dispersion processes inside the street canyon as the tree planting is arranged in a sheltered position with wind speeds being very small.
WT CONCENTRATIONSWT CONCENTRATIONS
relative deviations [%] in respect of tree-less street canyon
ResultsStreet Canyon with Tree Planting
CFD - WT CONCENTRATIONSCFD - WT CONCENTRATIONS
calculated concentrations relative deviations [%] in respect of measurements
Densely filled CrownDensely filled Crown
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
ResultsDiscussion and Statistical analysis
0.141.000.990.09small crown porosity (PVol = 96 %)
0.211.000.980.13large crown porosity (PVol = 97.5 %)
0.150.970.960.06tree-less street canyon
FBFAC2RNMSE
Statistical analysis (Chang, C. and S. Hanna, 2004):
•normalized mean square error (NMSE ≤ 4)•correlation coefficient (R)•fraction of predictions within a factor of two of observations (FAC2 ≥ 0.5)•fractional bias (−0.3 ≤ FB ≤ 0.3)
Comparison Wind Tunnel Measurements - Numerical Computations
CFD simulations are in general good agreement with wind tunnel experiments
Pollutant concentrations in proximity of the leeward wall are slightly underestimated in the numerical
simulations, while near the windward wall both slightly over- and underestimations are present
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
Summary and Conclusions
Influence of Trees on Flow Field and Pollutant Dispersion in Street Canyons
In-canyon air quality can be significantly altered by avenue-like tree planting
Air volume rotating in canyon vortex is reduced by the presence of tree plantings
Avenue-like tree planting cause overall increase in concentrations
Increases in concentrations at the leeward wall and decreases at the windward wall
The combination of experimental and numerical investigations in a novel
aspect of research can provide a strategy to investigate pollutant dispersion in
street canyons with tree planting, to obtain useful suggestions for assessment,
planning and implementation of exposure mitigation in urban areas.
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
Summary and Conclusions
-35%
+71%
H/W=1 –single tree row vs empty
H/W=0.5 –two tree rows vs empty
Relative deviation in concentration
-32%windward
+42%leeward
Concentration fields within street canyon depend on both street canyon aspect
ratio and tree planting configuration
Double tree rows is preferable to one row in the middle of the canyon
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
CODASC Databaseby Christof Gromke
Database containing concentration measurement data of street canyons with tree planting
It is meant for modelers (so all the necessary boundary conditions, like wind tunnel profile, geometry, etc.... are provided)
12th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes
Data from 40 experiments on street canyon/tree planting configurationsFree for downloadOnline next month at:
http://www.ifh.uni-http://www.ifh.uni-karlsruhe.de/science/aerodyn/CODASC.htmkarlsruhe.de/science/aerodyn/CODASC.htm
CODASC Databaseby Christof Gromke
THANK YOU FOR YOUR ATTENTION!